The exhaust gas discharged by the enterprises in the petrochemical industry and carbon fiber industry, such as acrylonitrile factory, synthetic rubber factory, plexiglass factory, carbon fiber factory, carbon factory and the like, is a highly toxic exhaust gas, which not only comprises hydrocarbons (CxHy), nitrogen oxides (NOx), but also comprises the cyanide-containing substances (R—CN) such as acrylonitride, hydrocyanic acid and acetonitrile, the direct discharge thereof will inevitably cause serious environmental pollution problems.
The exhaust gas treatment in current chemical industry in China is usually performed by direct discharge or direct combustion, and it cannot guarantee the requirement of the sustainable development of the enterprise. However, the catalytic combustion technology has the advantages such as low ignition temperature, recoverable waste heat, low energy consumption, good selectivity and the like, it must be the mainstream of the purification technology for the exhaust gas containing cyanogens in the petrochemical industry and carbon fiber industry in future.
With respect to the acrylonitrile production device, it usually performs production by the propylene-oxidation process, and the final discharged exhaust gas usually contains cyanide-containing substances such as acrylonitrile, acetonitrile and HCN, meanwhile, since the raw material gas propylene is impurity, the exhaust gas usually accompanies with propane, ethane, unconverted propylene and other hydrocarbons, and it further comprises certain amount of environmentally harmful components such as nitrogen oxides, carbon monoxide and the like. Since the complexity, low concentration, high toxicity, and large gas flow of the combustible exothermic components in the exhaust gas, the general technology adopted cannot meet the environmental requirements.
With respect to the acrylonitrile exhaust gas treatment technology, there are mainly two processes of thermal incineration and catalytic oxidation at present. The thermal incineration process has the following disadvantages: it needs to supplement a large amount of fuel, its operating cost is high, the cyanide-containing (R—CN) substances tend to be converted to Nx under high temperature, meanwhile the partial N2 in the exhaust gas is oxidized to form NOx under high temperature, which tends to cause the secondary pollution and needs subsequent additional NH3—SCR device to continue to remove, the requirements of the process parameters and removal effects are higher, and the process is more complex. However, when using catalytic oxidation process, under the action of the catalyst, the pollutants can be removed under the low temperature without supplementing additional fuel. Patent No. CN1903415 discloses that, the hydrocarbons in the exhaust gas from the acrylonitrile absorption tower are converted into carbon dioxide and water by using the catalytic oxidation process, but it does not account for the removal of NOx converted from R—CN by oxidation and NOx contained in the exhaust gas itself. Although the nitrogen in the exhaust gas will not continue to be converted to NOx, its NOx emission is also excessive. Patent No. CN101362051A discloses an exhaust gas treatment process for acrylonitrile device, using an oxidation reactor in which the precious metal honeycomb is used as a catalyst and an NH3—SCR reactor in which the vanadium/tungsten/titanium honeycomb ceramic is used as a catalyst, and the two reactors are combined to remove pollutants. The earlier stage reaction of the process is oxidation reaction, wherein the hydrocarbons, CO and R—CN are completely oxidized to produce CO2, H2O and NOx; and in the later stage, ammonia is used as the reducing agent to selective catalytic reduction of NOx. The process not only needs two reactors simultaneously, which has a long process flow and large equipment investment in earlier stage, but also expends a large number of ammonia during operation, meanwhile, R—CN in earlier stage is converted into NOx, which increases the load of the NH3—SCR apparatus, has a risk of NH3 leakage and overflow, and leads to excessive emissions of NH3 which resulting in a new source of contamination. Therefore, the process still has room for improvement.
The supported precious metal catalyst has a strong oxidizing property and a excellent removal effect on hydrocarbons, but it does not have a selectivity to nitrogen formation to the removal of the cyanide-containing substances (R—CN), hence, when the discharge value of the cyanide-containing substances in exhaust gas is large, it is prone to cause excessive emission of NOx in exhaust gas. For the high content of hydrocarbons and large heat release of the reaction, the temperature of the catalyst bed is prone to higher than 550° C., and the nitrogenous substances are very prone to converted to NOx under this temperature, thus, the content of NOx in the exhaust gas is higher when the precious metal catalyst is used. It is understood that, after the exhaust gas from the acrylonitrile production passing through the precious metal catalyst bed, the concentration of NOx in the exhaust gas is sometimes up to 1000 mg/m3, and it must add a NH3—SCR reactor subsequently for the removal of NOx. The high concentration of NOx in the exhaust gas not only increases the consumption of NH3, but also puts a higher requirement on the removal efficiency of SCR catalyst. Vanadium/tungsten/titanium catalysts also have the risk of harming the environment.